Facile Selective and Diverse Fabrication of Superhydrophobic

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Facile Selective and Diverse Fabrication of Superhydrophobic, Superoleophobic-Superhydrophilic and Superamphiphobic Materials from Kaolin Mengnan Qu,* Xuerui Ma, Jinmei He,* Juan Feng, Shanshan Liu, Yali Yao, Lingang Hou, and Xiangrong Liu College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China ABSTRACT: As the starting material, kaolin is selectively and diversely fabricated to the superhydrophobic, superoleophobicsuperhydrophilic, and superamphiphobic materials, respectively. The wettability of the kaolin surface can be selectively controlled and regulated to different superwetting states by choosing the corresponding modification reagent. The procedure is facile to operate, and no special technique or equipment is required. In addition, the procedure is costeffective and time-saving and the obtained super-repellent properties are very stable. The X-ray photoelectron spectroscopy analysis demonstrates different changes of kaolin particles surfaces which are responsible for the different superrepellency. The scanning electron microscopy displays geometric micro- and nanometer structures of the obtained three kinds of super-repellent materials. The results show that kaolin has good applications in many kinds of superwetting materials. The method demonstrated in this paper provides a new strategy for regulating and controlling the wettability of solid surfaces selectively, diversely, and comprehensively. KEYWORDS: superhydrophobic, superoleophobic-superhydrophilic, superamphiphobic, kaolin, underwater superoleophobic, topograph, self-cleaning

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INTRODUCTION The wettability is an important property of a solid surface, and the control of it is very valuable and interesting in both theoretical research and industrial applications. It is well-known that a superhydrophobic surface, which is one extreme case of wettability properties, has a water contact angle greater than 150° and a sliding angle less than 5°.1 The superhydrophobic surfaces were greatly anticipated to be used in applications such as self-cleaning coatings, anti-icing and antifogging surfaces, nonwetting fabrics, buoyancy and flow enhancement, etc.2−8 Due to these novel and important properties, the superhydrophobic surface has aroused considerable interests for many researchers inspired by the water-repellent nature of lotus leaves in recent years.9−11 By controlling the surface chemistry and surface microgeometries, numerous methods have been presented to fabricate the superhydrophobic surface with various materials.12−14 While there are also many liquid substances presented in nature, their physical and chemical properties are quite different from those of water. Similarly, a superoleophobic surface means that a surface has an oil contact angle higher than 150° and low contact angle hysteresis.15 Obviously, the superoleophobic surface is more important and interesting than the superhydrophobic surface in both fundamental research and industrial applications.16−19 Furthermore, the superamphipho© 2016 American Chemical Society

bic surface has better performance of wider super liquid repellency and lower surface adhesion than general superhydrophobic surfaces and superoleophobic surfaces in the respects such as nonfouling surfaces, expensive droplets transfer, and self-cleaning in oily conditions.20−26 In addition, the surfaces, which simultaneously possess superoleophobic and superhydrophilic properties, surely have unique property and better performance of selective liquid repellency than the ordinary superhydrophobic surface or superamphiphobic surface.27−31 This kind of material is ideal for oil−water separation, in which water permeates through the porous materials easily, whereas oil is retained. However, creating artificial superoleophobic or superamphiphobic surfaces has proven to be much more difficult than creating a superhydrophobic surface. The extreme difficulty in creating superoleophobic surfaces stems from the fact that oil molecules have a stronger attraction to the molecules of the solid surface than to each other due to its low surface tension (i.e., the adhesive forces are stronger than the cohesive forces).32−34 On the other hand, the water has a significantly higher surface tension than oils; a surface that is Received: August 31, 2016 Accepted: December 13, 2016 Published: December 13, 2016 1011

DOI: 10.1021/acsami.6b10964 ACS Appl. Mater. Interfaces 2017, 9, 1011−1020

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the selective and diverse fabricating processes for three kinds of super-repellent materials. Path A is used for superhydrophobic material, path B is used for the superoleophobic-superhydrophilic material, and path C is used for the superamphiphobic material.

In this work, we focus on a comprehensive technique to fabricate three different super-repellent materials from kaolin as the starting material. Remarkably, the superhydrophobic, superamphiphobic, and superoleophobic-superhydrophilic materials can be, respectively, obtained by diverse and selective modifications of kaolin particles. The most common kind of clay, kaolin, is adopted here because it is cheap, widely distributed on the earth, and widely used in many industrial applications. On the other hand, the super-repellent materials made from clay are rarely reported.47,48 The procedure is fairly facile to operate, and no special technique or equipment is required. Moreover, the procedure is cost-effective and timesaving and the as-fabricated super-repellent materials are very stable. To the best of our knowledge, this is the first time to use kaolin particles to selectively and diversely fabricate three kinds of super-repellent materials.

repellent to oil is usually also water-repellent. Thus, it is also difficult to create a surface with superoleophobicity and superhydrophilicity simultaneously. Heretofore, numerous studies have been reported for constructing superhydrophobic and superoleophobic surfaces, such as plasma treatment, deposition, self-assembly, sol−gel, spin-coating and spraying methods, and electrochemical methods.35−41 The superoleophobic-superhydrophilic materials have also been fabricated and applied, especially for the oil−water separation. The materials that were used to fabricate the surface morphology ranged from carbon nanotubes, nanowires, silicone nanofilaments, nanoparticles, and metal oxide nanorods to organic/ inorganic hybrids. According to previous research results, there are two ways to prepare liquid-repellent surfaces: one is to roughen the surface of low-surface-energy materials, and the other is to modify the rough surface with low-surface-energy materials. Additionally, some amphiphobic coatings with a low sliding angle have been created on smooth surfaces.42−45 It is important to note that most methods only provided a way to obtain a specific surface with only a single super-repellent property. Fluoroalkyl end-capped oligomer/talc composites were reported by Sawada and co-workers.46 The obtained fluorinated nanocomposites can provide the controlled surface active characteristics such as superoleophilic−superhydrophobic, superoleophobic−superhydrophilic, and superamphiphobic characteristics by controlling the molecules structures. It still remains a challenge to control the wettability of surface selectively and diversely.

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EXPERIMENTAL SECTION

Materials. Kaolin particles (before used, they were dried at 120 °C for 5 h to remove water) were purchased from Sinopharm Chemical Reagent Co., Ltd. Stearic acid was purchased from Zhengzhou Paini chemical reagent factory. Diethoxydimethylsilane (DEDMS) and triethoxymethylsilane (MTES) were all purchased from Sigma-Aldrich. Perfluorooctanoic acid (PFOA) and trichloro(1H,1H,2H,2Hperfluorooctyl)silane (PFOTS) were purchased from J&K Chemical Ltd. All of these chemicals were used as received. Fabrication of Superhydrophobic Material. Stearic acid (0.25 g) was added to a round-bottom flask containing 12 mL of ethanol under magnetic stirring. After completely dissolved, kaolin (5.0 g) was added to the solution, and ultrasonically dispersed for 20 min. Then, the flask was transferred into a preheated oil bath that was heated at a 1012


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Figure 2. (a) Photograph of water droplets (dyed with methyl blue) on the kaolin superhydrophobic material surface. (b) Profile of a water droplet on the superhydrophobic material surface having a contact angle of 158° ± 1°.

Figure 3. (a) Photograph of hexadecane droplets (dyed with methyl red) and water droplet (dyed with methyl blue) on the kaolin superoleophobicsuperhydrophilic material surface. (b) Photograph of underwater-hexadecane droplets (dyed with methyl red) are suspended from the superoleophobic-superhydrophilic material. (c) Contact angle measurement of different oily liquids in air and underwater. (d) The images show that the hexadecane droplet is nonadhesive on the underwater superoleophobic-superhydrophilic material (from left to right: hexadecane droplet was brought into contact with the surface, then the needle retracted from the surface). constant temperature of 100 °C, and refluxed for 2 h to form a superhydrophobic suspension. The resulting suspension was coated on a clean glass slide with a plastic head dropper, and the obtained specimen was dried at 120 °C for another 2 h. Finally, the material surface was rubbed gently. Fabrication of Superoleophobic-Superhydrophilic Material. Likewise, the PFOA (0.15 g) and sodium hydroxide (0.015 g) were added to a round-bottom flask containing 3.6 mL of ethanol under magnetic stirring, and the mixture was stirred until PFOA and NaOH fully dissolved. Subsequently, kaolin (1.5 g) was added to the solution. The reaction was conducted by ultrasonic dispersion for 30 min. Then, the flask was transferred into a preheated water bath that was heated at a constant temperature of 52 °C. Subsequently, 0.1 mL of MTES and 0.05 mL of DEDMS were added in the mixture. The reaction was conducted by magnetic stirring for 3 h. The obtained suspension was coated on a clean glass slide with a plastic head dropper. The resulting specimen was dried at 80 °C for 2 h. Finally, the material surface was rubbed gently. Fabrication of Superamphiphobic Material. First of all, a fluoro-containing homogeneous solution (0.1 mL of PFOTS and 3 mL of acetone were mixed) was fabricated. Kaolin (1.5 g) was added into anhydrous ethanol (2 mL) under magnetic stirring to form a homogeneous suspension, and the fluoro-containing (2 mL) solution was then added. After 20 min of ultrasonic dispersion, the reaction was conducted by stirring and heating in a water bath at 50 °C for 3.5 h. The resulting suspension was coated on a clean glass slide with a

plastic head dropper, and the obtained specimen was dried at 120 °C for 2 h. Finally, the material surface was rubbed gently. Characterization. Static contact angle measurements were carried out using an optical contact angle goniometer (JC2000DM, China), and the average of five readings was used as the final contact angle of each sample. The surface morphologies of the materials were investigated by scanning electron microscopy (SEM, JEOL JSM6460LV). All the samples were coated with gold clusters before the SEM investigation. The microscale lateral resolution images and the surface roughness of the as-prepared materials surfaces were obtained by using an Rtec Universal 3D profilometer (Rtec-Instrument, USA). The surface chemical compositions were investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha, USA). Infrared spectra were recorded by using Fourier transform infrared spectroscopy (FTIR, PerkinElmer System 2000). Different spectra were collected over the range of 500−4000 cm−1. A transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN) was used to observe the bare and modified kaolin particles of materials. The samples for TEM were prepared by scraping particles from the sample and ultrasonically dispersed in ethanol for 30 min. The particle size distributions of the original kaolin particles and the three modified kaolin particles were analyzed by using a laser molecule distribution tester (LS230/VSM+, Coulter, USA). The samples were prepared by grinding particles of materials and ultrasonically dispersed for 20 min in ethanol. 1013


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Figure 4. (a) Photograph of water (dyed with methyl blue) and different oil liquids (dyed with methyl red) on the kaolin superamphiphobic material surface. (b) Contact angle measurement of different liquids on the superamphiphobic material surface.

Figure 5. XPS spectra of the as-prepared (a) superhydrophobic material, (b) superoleophobic-superhydrophilic material, and (c) superamphiphobic material.

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RESULTS AND DISCUSSION It is generally believed that the functional groups of the kaolin surface are −Si(Al)−OH, −Si−O−Al−, and −Si(Al)−O.

Figure 7. FTIR spectra of (a) bare kaolin particles, (b) superhydrophobic material, (c) superoleophobic-superhydrophilic material, and (d) superamphiphobic material.

Figure 6. C 1s XPS high-resolution spectra with fitting of (a) superhydrophobic material, (b) superoleophobic-superhydrophilic material, and (c) superamphiphobic material. Si 2p XPS highresolution spectrum with fitting of (d) superamphiphobic material.

surface-energy alkyl groups. The carboxyls at the end of stearic acid molecules combined with the hydroxyl of the kaolin surface by the covalent bonds formed via the esterification reaction. This surface reaction made the kaolin particles surrounded with the hydrophobic group of −CH2− and −CH3, as shown in path A (Figure 1). A layer of stearic acid molecule membrane was then formed on the surface of kaolin particles and made these particles possess enough low surface energy. After these modified kaolin particles were closely packed on the substrate, the materials with micro- and nanostructures have been fabricated, and then the good superhydrophobicity has been achieved. Figure 2a is the image of water droplets with different sizes on this stearic acid modified kaolin material surface. Figure 2b displays that the as-prepared material has good water repellency and the water contact angle is 158° ± 1°. In addition, the water droplets

These active sites are the basis of these surface modifications. Moreover, the kaolin particles have diameters of about several hundred nanometers and they were connected to each other firmly and closely packed. It is very appropriate and convenient to construct the required micro- and nanoscale structures by the accumulation of particles on a substrate. Due to the reactive groups and the micro- and nanoscale structures mentioned above, kaolin particles were adopted here. The processes to prepare the three different super-repellent materials from kaolin are schematically illustrated in Figure 1. First, the stearic acid was used because of its functional carboxyl groups and low1014


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Figure 8. SEM images at different magnifications of (a, b) superhydrophobic material, (c, d) superoleophobic-superhydrophilic material, and (e, f) superamphiphobic material. The scale bars represent (a, c, e) 5 μm and (b, d, f) 1 μm, respectively.

Figure 9. 3D profile images of as-prepared three material surfaces: (a) for superhydrophobic material surface, (b) for superoleophobicsuperhydrophilic material surface, and (c) for superamphiphobic material surface.

PFOA modified kaolin material surface, the superoleophobicity to different kinds of oil was also measured (Figure 3c). The oil contact angles of the white oil, castor oil, rapeseed oil, and olive oil are 152, 150, 152, and 151°, respectively, confirming the good oil repellency. Interestingly and importantly, although the PFOA modified kaolin material shows good superoleophobicity to many kinds of oil droplets, it does not show any hydrophobicity at all. It demonstrates excellent superhydrophilicity that the water droplet on this material surface can spread instantly within 2 s (CAwater ≈ 0°) and then completely permeate into the material surface, as shown in Figure 3a. It means this as-prepared material has simultaneous superoleophobicity and superhydrophilicity. It is worth noting that Zhao et al. reported a smart superamphiphobic coating from the silica nanoparticles/heptadecafluorononanoic acid-TiO2 composites.49 The obtained coating exhibits stable superhydrophobicity in air while it shows superhydrophilicity upon ammonia exposure. The reason is probably that the kaolin

can easily roll down the sample surface, implying a low contact angle hysteresis. It has been reported that the lowest-surface-energy groups in monolayer films have an order of −CH2− > −CH3 > −CF2− > −CF2H > −CF3. Thus, PFOA is fairly suitable and adopted here for lowering the surface energy due to its high content of −CF3, −CF2−, and the carboxyl. According to the path B (Figure 1), kaolin particles were modified with PFOA by esterification reaction to obtain the superoleophobic material. In addition, MTES and DEDMS were selected and added here in order to enhance the rigidity of material through polymerization of MeSi(OEt)3 and Me2Si(OEt)2. After this surface chemical modification and the subsequent fabrication of micro- and nanostructures on the substrate, the resulting material displays excellent oil repellency. Figure 3a is the hexadecane droplets dyed with methyl red on the PFOA modified kaolin material surface having a contact angle of 151° ± 1°. To verify the applicability of the oil repellency of the 1015


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Figure 10. TEM images of (a) bare kaolin particles, (b) stearic acid modified kaolin particles, (c) PFOA modified kaolin particles, and (d) PFOTS modified kaolin particles. The scale bars all represent 200 nm.

molecules on the material surface, and the water film reduces the contact between the oil and the material surface. This underwater oil-shielding ability of the as-prepared material was also qualitatively demonstrated by observing the underwater oil adhesion.19 Figure 3d shows a 10 μL hexadecane droplet was used as a testing probe and was controlled to approach, contact, and retract from the material surface placed underwater. The result demonstrates that the oil droplet maintained its spherical shape throughout, and no residual oil remained on the material surface. These results clearly indicate that the as-prepared material has an excellent superoleophobicity-superhydrophilicity and underwater oil-shielding ability, which is very attractive for the real applications. The schematic illustration of preparing the superamphiphobic kaolin material has been shown in path C (Figure 1). When PFOTS was added to the mixture solution, the hydrolysis and condensation of PFOTS took place. The three Si−Cl groups of PFOTS are hydrolyzed to Si−OH groups. Then, the fluoroalkyl groups have been attached to the surfaces of kaolin nanoparticles and a three-dimensional network structure formed by the interaction and condensation. After these functionalized kaolin particles have been accumulated on the substrate, the obtained material displays super-repellency toward both oil and water droplets. Figure 4a is the image of different kinds of liquids on the as-prepared superamphiphobic material. The superamphiphobicity of the material to different kinds of liquids was also evaluated by measuring the contact angle. The liquids contact angles to the water, white oil, olive oil, rapeseed oil, and castor oil are 155, 152, 151, 152, and 153°, respectively, as shown in Figure 4b. The material coating can be used on a

Figure 11. Particle size distribution of (a) original kaolin particles, (b) PFOTS modified kaolin particles, (c) PFOA modified kaolin particles, and (d) stearic acid modified kaolin particles.

particle, as a kind of natural mineral, differs greatly to TiO2 nanoparticles. Remarkably, when this superoleophobic-superhydrophilic material is submerged in water, the material surface still keeps nonwetting to hexadecane with the oil contact angle of about 153° (Figure 3b). Furthermore, the underwater oilrepellent property of the material to different oily liquids was also evaluated. The bar charts in Figure 3c present the underwater contact angles of the resulting material to different kinds of oil. The underwater oil contact angles to the white oil, castor oil, rapeseed oil, and olive oil are 154, 153, 155, and 152°, respectively. It reveals that the underwater contact angles have slightly increased than in air. This is probably because the superhydrophilicity of the material causes a layer of water 1016


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Figure 12. Self-cleaning performance of (a) superhydrophobic, (b) superoleophobic-superhydrophilic, and (c) superamphiphobic materials are tested with methyl blue powder.

resulting in the superhydrophilicity. Because the oil droplet is apolar and the interaction is weak enough, oil droplets kept their high contact angle and sphere shape on the surface. The two reasons account for the simultaneous superoleophobicity and superhydrophilicity and result in the corresponding material. To verify the mechanism of this superoleophobicsuperhydrophilic property, the PFOA modified kaolin particles have been ultrasonically washed with water many times to remove the PFOA through hydrolysis. The reconstructed material has completely lost this superoleophobic-superhydrophilic property and can be easily wetted by oil and water, due to the intrinsic hydrophilicity of the kaolin particle surface. Figure 6c displays the C 1s high-resolution spectrum of the superamphiphobic material. The C 1s peaks located at 291 and 293.4 eV are assigned to the carbons of −CF2− and −CF3−, respectively. The peak at 284.6 eV is assigned to the carbon atom of C−C/C−H. The Si 2p peak of the superamphiphobic coating can be deconvoluted into two components with binding energies at 103 and 102 eV (Figure 6d), which can be attributed to Si−O and Si−C bonding, respectively. These results imply that the Si−O−Si threedimensional network structures and long-chain fluoroalky have successfully assembled on the surfaces of the kaolin particles. To further study the diverse modifications of kaolin particles, FTIR of the bare kaolin particles, steric acid modified kaolin particles, PFOA modified kaolin particles, and PFOTS modified kaolin particles has been carried out and compared (Figure 7). Figure 7a presents the FTIR spectrum of bare kaolin particles. Compared with Figure 7a, Figure 7b reveals that three characteristic absorption peaks have appeared in the spectrum of steric acid modified kaolin particles. The adsorption vibrations at 2910 and 2849 cm−1 are attributed to C−H asymmetric and symmetric stretching vibrations, and the peak at 1705 cm−1 is assigned to the ester base (−COO).51 These obvious characteristic peaks demonstrate that the esterification reaction has occurred by the carboxyl at the end of stearic acid combined with the hydroxyl of the kaolin surface. FTIR of the superoleophobic-superhydrophilic kaolin particles modified by PFOA is shown in Figure 7c; the peaks at 1077 and 887 cm−1 are attributed to the C−F groups of the PFOA.52 Meanwhile,

variety of substrates, which can greatly expand the scope of applications. The unmodified kaolin particles are intrinsic hydrophilic, while they can be selectively converted to the superhydrophobic, superoleophobic-superhydrophilic, and superamphiphobic materials by surface modification with steric acid, PFOA, and PFOTS, respectively. XPS was carried out to investigate the key information concerning the great changes of the kaolin particle surfaces. Figure 5a presents the XPS survey spectra of the stearic acid modified kaolin materials. It displays a strong signal of C 1s, which confirms that the kaolin particles are covered with a layer of stearic acid membrane. The highresolution C 1s spectrum of the stearic acid modified kaolin material is presented in Figure 6a. The major peak at 284.5 eV corresponds well to the C−C/C−H bond. The peak at 288.4 eV is attributed to the −COO− bond, indicating that esterification reaction has successfully occurred. Figure 5b,c presents the XPS survey spectra of the as-prepared superoleophobic-superhydrophilic material and the superamphiphobic material, respectively. Two strong fluorine peaks located at 686.7 eV and the peaks of Si, C, O, and Al are observed in both spectra. The results demonstrate that the kaolin particle surfaces are covered with the fluoroalkylsilane film which greatly reduced the free energy of the surfaces. The corresponding C 1s high-resolution spectrum of superoleophobic-superhydrophilic material can be curve-fitted into five carbons with different environments (Figure 6b) where they are identified as: C−C/C−H (284.8 eV), −COO− (288.5 eV), −COOH (290 eV), −CF2− (292.2 eV), and −CF3 (294.3 eV).50 The existence of the −CF2− and −CF3 groups has greatly reduced the material surface energy and bestowed a good superoleophobicity on the material. However, the ester group formed by PFOA and kaolin is unstable because of the electron-withdrawing inductive effect of the fluorine atom. A reversible hydrolysis easily occurred, leading to that there are many hydrophilic carboxyls existing on kaolin particles. Due to the fact that the water molecule is polar and the carboxyl is also polar and hydrophilic, there are great interactions between the carboxyls and the water molecules. This interaction is so strong that the water droplets can easily permeate into the material, 1017


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ACS Applied Materials & Interfaces the characteristic peaks at 1698 and 1633 cm−1 are assigned to the stretching vibration of CO (ester carbonyl and carboxyl groups), respectively. Because the ester group formed by PFOA and kaolin is easily hydrolyzed, it is worthwhile noting that the −COOH vibration of PFOA modified material at 3500 cm−1 is presented in Figure 7c, which leads to the superhydrophilicity of the material surface. The absorption vibrations at 1099 cm−1 in Figure 7d are broader and stronger than the peaks in Figure 7b,c, owing to the overlapping peaks of the Si−O−Si antisymmetric stretch vibration and C−F absorption.53 The adsorption vibration at 896 cm−1 is attributed to Si−O symmetric stretching vibrations.54 These spectra demonstrate the successful reaction of esterification (Figure 7b,c) and the long-chain fluoroalkyl functionalized kaolin particles with Si− O−Si three-dimensional network structures are successfully obtained (Figure 7d).17 The super-repellency property is commonly believed to be due to the presence of synergistic binary geometric structures at micro- and nanoscale, which reduce the energy of the surface.55 After the mixture was dropped on the substrate and dried out, the modified kaolin nanoparticles are interwoven and accumulated on the surface irregularly, thus forming rough porous structures at microscale. The geometric microstructures of the as-prepared three types of materials formed on a substrate have been observed carefully by SEM. Figure 8a shows the typical rough morphology of the superhydrophobic material and reveals that there are some rodlike structures distributed randomly across the material surface, which further increases the surface roughness. Figure 8b is the magnified image of the rough structures and reveals that kaolin particles are converged together to form the micro- and nanoscale hierarchical structures. This irregular accumulation of kaolin particles might be vital for the superhydrophobic surface to obtain the hierarchical roughness. The micro- and nanoscale porous structures have already trapped enough amount of air to prevent the penetration of the water droplet into the cavities, which bestowed the superhydrophobicity on the surfaces. There are also some voids in Figure 8a,b, which could afford not only the superhydrophobic but also the superoleophilic characteristic on the surface. This characteristic is owing to oils having a lower surface tension (ca. 20 mN/m) than that of water. Although superamphiphobic coatings have been successfully created on smooth surfaces,42−44 micro- and nanoscale structures and low surface energy are still two crucial factors for a superoleophobic surface. Panels (c) and (d) in Figure 8 are the SEM images of the superoleophobic-superhydrophilic material. A typical rough and porous surface can be clearly observed from the SEM image (Figure 8c). Figure 8d is the magnified image of panel (c) and shows that there are protrusions with several micrometers on the top of the microstructure. The SEM images of the as-prepared superamphiphobic materials, as shown in Figure 8e,f, are very similar to the superoleophobic-superhydrophilic material surfaces. Figure 8e also shows the typical rough surface morphology. Figure 8f reveals that there are microparticles with a diameter of about several hundred nanometers on the top of apophyses, and the nanoparticles protruded to form nanometer scale structures. On the basis of recent study, these structures can dramatically increase the trapped air within the grooves, which can effectively prevent the liquid from penetrating into the surface.56,57 The multilevel micro- and nanocomposite structures combined with the low surface energy imparted by

the fluorinated kaolin particles lead to outstanding superamphiphobicity. Enhancing the surface roughness is generally very important for creating the highly super-repellent surface, besides the lower surface energy. To investigate the surface roughness of the three as-prepared materials more deeply, they were also quantitatively analyzed by a 3D profilometer. The 3D topography image of the superhydrophobic surface is shown in Figure 9a. It can be seen that there are many microsized mastoid structures on the surface, which are familiar to the microstructures of the lotus leaf. The corresponding surface roughness (Ra) is 14.8 μm, which suggests that the as-prepared material surface modified by stearic acid has the necessary roughness for superhydrophobicity. Figure 9b is the 3D profile image of the superoleophobic-superhydrophilic material surface. It shows that there are many uniform acupuncturing bumps on surfaces, and its surface roughness is 1.93 μm. The 3D profile image of the superamphiphobic material surface, as shown in Figure 9c, is similar to the superoleophobicsuperhydrophilic material surface. Figure 9c indicates that these kaolin particles are packed more tightly and the surface roughness is 3.67 μm. Figure 9b,c also demonstrates that these closely packed kaolin particles have afforded enough necessary roughness, leading to the remarkable super-repellency. The TEM is also employed to analyze the morphology of the bare kaolin particles and modified kaolin particles, as shown in Figure 10. Figure 10a reveals that some of bare kaolin particles are congregated together and formed a microstructure cluster, which have the diameter of about several micrometers. Panels (b)−(d) in Figure 10 are TEM images of the kaolin particles modified with stearic acid, PFOA, and PFOTS, respectively. These images also show that there are some irregular microand nanoscale particles attached to these larger kaolin particles. They reveal that some particles are nanoscale having a diameter of about 200 nm, and these modified particles also congregate together firmly and form a microstructure cluster, which has the diameter of about 1 μm. Such an aggregation phenomenon is owing to longer chains of kaolin surfaces and the agglomeration of solid particles themselves. These nanoparticles are accumulated and joined so firmly that they cannot be disconnected separately by ultrasonication. Both the nanoscale particle size and the rough cluster structures have contributed to build the micro- and nanoscale hierarchical structures, potentially making the materials super-repellent. Figure 11 displays the particle size distribution of the original kaolin particle and the three kinds of modified kaolin particles. It can be seen that the shapes of the four curves are very similar, and it reveals that the four particle size distributions have not obviously changed even though they were modified with different reagents. Furthermore, the curves shows that the main sizes of the four kinds of kaolin particles are 2 μm, 7 μm, and 12 μm, respectively. It is owing to the kaolin particle aggregation, which could not be disconnected separately by ultrasonic dispersion and contributed to build the microscale structures. The self-cleaning ability accelerates the applications of liquidrepellent materials in real environments. To evaluate the selfcleaning ability of the three different kinds of materials, they are placed separately in a watch glass with a slight inclination angle.58 Methyl blue powders are used as the sample dust and placed on the three materials surfaces, respectively. The superhydrophobic material surfaces are subjected to two or three falling water droplets. The material surface shows sharp self-cleaning ability by allowing water droplets to carry the 1018


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powder away from their rolling and bouncing path, as shown in Figure 12a. Similarly, the rolling oil droplet that fell on the superoleophobic-superhydrophilic material surface is also able to pick up the powders without influence on its motion (Figure 12b). Compared with superhydrophobic and superoleophobicsuperhydrophilic materials, both water and oil droplets can get rid of powders which are placed on the surface of the superamphiphobic material (Figure 12c). According to the selfcleaning ability of these materials simply investigated, it can be concluded that the three kinds of super-repellent materials can achieve an effective self-cleaning process under different environments.

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CONCLUSIONS In summary, we developed a selective and diverse method to, respectively, fabricate superhydrophobic, superoleophobicsuperhydrophilic, and superamphiphobic materials from kaolin. The as-prepared superoleophobic-superhydrophilic material showed good superoleophobicity to many kinds of oil droplets both in air and underwater, while the water droplet on its surface can quickly spread and permeate into the material within 2 s. With this superoleophobic-superhydrophilic and underwater oil-shielding ability, the material is sure suitable for the separation of different oil/water mixtures. The as-prepared superhydrophobic and superamphiphobic materials showed good and corresponding liquid repellency. With these superrepellent and self-cleaning abilities, these materials will accelerate and expand the applications in superamphiphobic materials research. The method has provided a good example to achieve super-repellent materials with clay, which is abundant on the earth. Moreover, this method is fairly facile to operate and no special technique or equipment is required, and it can be easily applied to different types of clay. In addition, the procedure is cost-effective and time-saving and the as-fabricated super-repellent surfaces are very stable. Finally, we greatly hope the method demonstrated in this paper will provide a strategy for regulating and controlling the wettability of solid surfaces selectively, diversely, and comprehensively, and also for the super-repellent material research.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Q.). *E-mail: [email protected] (J.H.). ORCID

Mengnan Qu: 0000-0002-0684-4162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grant Nos. 21473132, 21373158), the Shannxi Science and Technology Department (Grant Nos. 2014JM2047, 2013KJXX-41) for continuing financial support.

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REFERENCES

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Facile Selective and Diverse Fabrication of Superhydrophobic

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